Casting of metals has long been done by the use of a continuous caster such as that shown in U.S. Pat. Nos. 2,790,216; 4,054,173; or 4,303,181. These patents are incorporated herein by this reference. Such a continuous caster comprises a pair of rotating water cooled rolls. Molten metal is fed into the bite of the rolls just prior to the line of closest approach of the two rolls. Heat is rapidly extracted from the metal, which results in the metal freezing. The caster rolls form a rotating mold for the metal. The frozen metal then continues through the gap between the rolls for hot working the metal. It is important to be able to control the gap between the casting rolls, since that controls the thickness of the casting.
There are important considerations in continuous casting of metal which are not present in mere rolling of metal. The location of the point where the last o the molten metal freezes has an influence on the properties of the sheet of metal cast in the roll caster. If the metal freezes early, that is, with a relatively thicker cross section, there is more hot reduction of the solid metal. This is seen in the separating force between the rolls and in the torque on the motors which rotate the rolls. The increased separating force elastically deforms the mill frame and increases the separation between the rolls, leading to thicker metal cast. Conversely, if the metal freezes relatively late, there is concomitantly less deformation of the metal after freezing and less grain refinement.
Thus, it is desirable to maintain the locus of freezing of the metal in a relatively constant position between the rolls. The locus of freezing is strongly influenced by roll speed as well as roll gap. Slowly rotating rolls leave the metal in contact with the rolls longer, causing freezing a greater distance from the nip, and faster rotational speeds may leave metal unfrozen as it passes through the nip, a severe problem. The roll speed therefore affects sheet quality as well as production rate.
Either or both of the rolls in a roll caster may be slightly eccentric in the bearings so that the gap between the rolls periodically varies as the eccentricities engage the metal being cast. This leads to an undulating thickness in the cast sheet. This can be seen in variations in separating force or roll motor torque, as well. This may be a particular problem in roll casting machines because of the way the rolls are formed.
Typically, the rolls are in the form of a central core with water cooling passages around its perimeter. A relatively thin shell of steel is heat shrunk onto the core to form the surface that contacts the metal being cast. The shell is carefully ground to be concentric with its bearings. However, as the roll is used, the shell may creep around the core so that its outer surface is no longer concentric with the bearings.
In one type of casting machine, the "profile" or cross section of the sheet being cast may be controlled by axially shifting one of the rolls. In such a machine, each of the rolls is non-cylindrical with a slight S-shaped profile complementary to the profile of the other roll. When one of the rolls is shifted axially, the sheet may be cast with a slight crown for guidance in subsequent rolling operations, or may be cast without a crown for subsequent rolling into foil, for example.
In such an embodiment, thickness measurement on the chocks or roll necks may not indicate minor changes in thickness occurring as a result of roll shifting. Further, mechanical arrangements for measuring thickness do not fit well on such a casting machine.
Most prior techniques for controlling a continuous roll caster have relied on measurements of thickness of the finished sheet, and can therefore maintain a general thickness tolerance without being able to control the undulations in thickness which may occur due to roll eccentricities and non-uniform position of the locus of freezing. This is due to measurement of sheet thickness at a location "downstream" from the rolls where the metal is frozen and hot worked. Such a locus of measurement inherently has a time lag so that corrections of metal thickness are made after the errors may have persisted for a period. This leads to non-uniformities in sheet thickness which are undesirable.
A system for controlling the roll gap in a casting machine is described in U.S. Pat. No. 4,678,023 to Knapp. This method directly measures the gage of the casting. The disclosure indicates that this can be done by sending X-ray or Beta ray signals through the casting itself, and then measuring the strength of the remaining signal. However, the Knapp method is dependent on the composition of the metal being cast. Any change in the composition changes the absorption properties of the metal. This requires that new reference values be determined to correspond to that particular composition.
In addition, there is an inescapable time delay when measuring the metal some distance from the rolls. What is actually being measured is the gap that used to be between the rolls. Since constant changes in the position of the rolls is possible, this time delay greatly reduces the effectiveness of a monitor which measures the metal directly. Clearly, other methods of directly measuring the casting itself also suffer from such disadvantages.
Techniques used for measuring the roll spacing in rolling mills are not necessarily suitable for measuring the gap between the rolls in a casting machine. For example, a common instrument for measuring the roll spacing in a rolling mill employs what are effectively mechanical calipers engaging a measurement band on the roll necks concentric with the roll surfaces. As has been mentioned, the rolls in a caster may not remain concentric. Further, the space required for these mechanical devices prohibits usage for measuring the roll gap directly in a casting machine. This is particularly true in a machine with axial shifting of the rolls.
It is desirable to provide a technique which directly monitors the thickness of the metal while still between the casting rolls and is independent of the properties of the metal being cast. It is also desirable to remove the monitoring equipment from the immediate area of the casting rolls to protect it from the hostile environment which often results there.
It is also desirable to measure metal thickness while between the rolls at the same time measurements are made of separating force and/or roll torque. It is desirable to be able to control roll gap in real time along with roll speed.